The present invention relates to a curable composition for top coating, and more particularly to a curable composition which has an improved excellent acid resistance in addition to excellent other properties such as water resistance, scratch resistance, appearance and thermosetting property and which is suitable as top coat for interior and exterior of buildings and for various articles such as automobiles, industrial equipments, steel furnitures, household electric appliances and plastic products, particularly suitable as top coat for automobiles.
Paints containing a melamine resin such as alkyd-melamine resin or acrylic melamine have hitherto been mainly used for coating automobiles, interior and exterior of buildings, industrial equipments, steel furnitures, plastic products and the like. However, these melamine resins involve the problems that harmful formalin generates when curing the paints, and that the cured coating films are damaged by acid rain because they are poor in acid resistance. In particular, the problem of acid rain resulting from air pollution assumes a serious aspect in recent years, and there is observed a phenomenon that etching, whitening or spotting occurs in the coating films.
It is known that vinyl copolymers having a hydrolyzable silyl group or their blends with acrylic polyols are curable and useful for the production of paints having a good acid resistance and a good weatherability, for instance, as disclosed in Japanese Patent Publication Kokai No. 54-36395, No. 58-157810 and No. 01-141952.
These polymers or polymer blends have the feature that since hydrolyzable silyl groups and alcoholic hydroxyl groups form stable siloxane bonds or siloxy bonds to cure, there can be formed coating films having good acid resistance and weatherability as compared with melamine resins such as acrylic melamine resin and alkyd melamine resin. Further, when the hydrolyzable silyl groups are those shown by the formula (II): ##STR1## wherein R.sup.3 is an alkyl group having 1 to 10 carbon atoms, R.sup.4 is H or a monovalent hydrocarbon group selected from an alkyl group having 1 to 10 carbon atoms, an aryl group or an aralkyl group, and b is 0, 1 or 2, the coating films are particularly superior in acid resistance and weatherability, since the hydrolyzable silyl groups are introduced into the polymer main chain through Si-C bond.
The blend systems of a hydrolyzable silyl group-containing vinyl copolymer and an acrylic polyol have the advantages, as compared with the use of the vinyl copolymer alone, that the thermosetting property and appearance of the obtained films are better and that reduction of cost for preparing curable coating compositions is possible because the amount of a compound having a hydrolyzable silyl group which relatively expensive raw material, can be reduced. However, these blend systems are not always satisfactory in water resistance and acid resistance. Further, in order to achieve a good compatibility between the hydrolyzable silyl group-containing vinyl copolymer and the acrylic polyol, the molecular weights of the respective polymers, the composition of monomers and the blending ratio of the polymers are somewhat restricted.
In order to solve these problems, it is proposed to use a vinyl copolymer having both a hydrolyzable silyl group and an alcoholic hydroxyl group in the same molecule, e.g. a copolymer of a vinyl monomer having a hydrolyzable silyl group and a vinyl monomer having an alcoholic hydroxyl group. When this polymer is used in paints, the water resistance and acid resistance of coating films are improved to some extent. However, further improvement in acid resistance is demanded, since in recent years occurrence of spot in coatings owing to acid rain is in question in North America and Europe and particularly since coatings of automobiles suffers damage from acid rain.
It is an object of the present invention to provide a curable resin composition having an improved excellent acid resistance.
A further object of the present invention is to provide a coating composition, particularly a coating composition suitable as a top coat which has excellent properties such as acid resistance, water resistance, appearance, weatherability, abrasion resistance, scratch resistance, thermosetting property and other properties required for coatings.
These and other objects of the present invention will become apparent from the description hereinafter.